Light olefins, defined herein as ethylene, propylene, and mixtures thereof, serve as feeds for the production of numerous important chemicals and polymers. Light olefins traditionally are produced by cracking petroleum feeds. Because of a limited supply of competitive petroleum feeds, the opportunities to produce low cost light olefins from petroleum feeds are limited. Efforts to develop light olefin production technologies, based on alternative feeds, have increased.
An important type of alternate feed for the production of light olefins are oxygenates, such as, for example, alcohols, particularly methanol and ethanol, dimethyl ether, methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate. Many of these oxygenates may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates show promise as economical, non-petroleum sources for light olefin production.
Typically, oxygenates are converted to an olefin product through a catalytic process. The conversion of a feed containing oxygenates is usually conducted in the presence of a molecular sieve catalyst. Although ZSM-type molecular sieves and other molecular sieves may be used for the production of olefins from oxygenates, research has found silicoaluminophosphate (SAPO) molecular sieves to be of particular value in the catalytic process.
While SAPO molecular sieves are thought to be the most useful, synthesis of this type of catalyst is expensive because of the low yield of molecular sieve provided by the reaction mixture used to formulate this type of molecular sieve. In a SAPO synthesis procedure, a silica source, an alumina source, a phosphorous source and a templating agent are combined to form a reaction mixture. The SAPO molecular sieve is then crystallized over a period of time, typically a period of several hours to is several days, from the reaction mixture.
The synthesis of SAPOs is sensitive to small variations in the reaction mixture composition and reaction mixture preparation. These sensitivities vary from one type of SAPO to another. One critical parameter of the synthesis procedure is the pH of the reaction mixture. At the start of a SAPO synthesis, the reaction mixture, sometimes referred to in the art as the “final” reaction mixture, has an initial pH. As the synthesis proceeds, the pH of the reaction mixture increases. It has been found that this increase in pH makes it difficult for the SAPO molecular sieve to crystallize from the reaction mixture even in the presence of excess quantities of the silica source, the alumina source, the phosphorous source and the template, and this pH increase eventually causes the synthesis reaction to cease.
Romanian Patent No. 114,524 B1 describes a process for forming a SAPO molecular sieve, particularly for forming SAPO-34. In the process disclosed in the '524 patent, a solution of tetraethylammonium phosphate with a concentration of 25% is prepared using a conventional method from triethylamine, ethyl bromide and 73% concentrated phosphoric acid. Hydrated alumina with an Al2O3 content of 65%, of which 40% is bayerite, is suspended in demineralized water and introduced, under agitation, into a zeolitization autoclave after the solution of tetraethylammonium phosphate was introduced to the autoclave. Under continuous agitation, a silica sol which is stabilized with ammonia and which has a content of 28% SiO2 is introduced to the autoclave. The pH of the resultant suspension is then adjusted to 6.3-6.5 with phosphoric acid.
The zeolitization process is conducted in six successive steps. In the first step, 15% of the entire charge is introduced into an autoclave. The temperature is then increased to 198-205° C. and maintained at that point for 20 hours. The autoclave is cooled to 30-40° C. and an additional quantity of the suspension is introduced to the autoclave. After addition of an additional amount of the suspension, the process is resumed at 198-205° C. The operation is repeated for an additional period of five hours. The entire zeolitization process lasts for 100 hours. This time period includes the steps of cooling and heating which last for 2-3 hours each.
WO 99/19254 describes a method for making molecular sieves comprising SAPO-44. In a preferable version of the process for making SAPO-44, the pH of the final reaction mixture (containing a silicon component, a phosphorous component, an aluminum component and a template) is maintained in the range from about 5.5 to about 8.5, preferably from about 6 to about 8. This reference teaches that the pH value of the final reaction mixture may be adjusted, if desired, by either adding an appropriate amount of a base, such as ammonia/ammonia hydroxide, to increase the pH, or an appropriate amount of a suitable inorganic or organic acid, such as phosphoric acid, HCl, acetic acid, formic acid, CO2 and others, to decrease the pH.
With the aforementioned methods and other methods currently in use the art, an initial pH adjustment to the reaction mixture helps to establish appropriate conditions for the formation of a SAPO molecular sieve. These methods, however, do not alleviate or prevent the pH increase which occurs as the synthesis reaction continues. As discussed above, this pH increase drives the synthesis reaction to completion even in the presence of excess building materials for the sieve, often providing low yields of the molecular sieve. Thus, a need exists in the art for improved methods for synthesizing SAPO molecular sieves.